- © The Mineralogical Society Of America
Identifying the mechanisms that drove the evolution of planetary volatiles from primordial to present-day compositions is one of the classic challenges in the planetary sciences. The field bristles with models of one type or another, but none are without difficulties. Efforts to understand the histories of volatile species in the atmospheres and interiors of the terrestrial planets have concentrated on their noble gases, free of the entanglements of chemical interaction, as evolutionary tracers. The elemental and isotopic compositions of these minor constituents are rich in clues to the chemical characteristics of their source reservoirs, the physics of their evolution, and the nature of the astrophysical and planetary environments in which they evolved. Most workers would agree that atmospheric mass distributions of the nonradiogenic noble gases were probably established very early, through the action of processes operating before, during, or shortly after planetary accretion. But beyond this there are no certainties as yet on the specifics of sources or mechanisms.
The question of past and present volatile inventories on and in the terrestrial planets is intrinsically interesting in the more general context of the evolution of the planets themselves. For each individual body it crosscuts issues relating to atmospheric origin and compositional history, emergence of a coupled atmosphere-surface system and climate evolution, surface morphology and its record of past geological processing, and planetary rheology, differentiation, and degassing history. But the volatile problem extends well beyond its relationship to the initial state of a particular planet and the mechanisms that drove it down its specific evolutionary track. Attempts to decipher planetary histories in the broader context of the evolution of the solar system as a whole are focusing more and more attention on the sources and processing of volatiles in the primordial solar accretion disk, in primitive meteorites, and in the terrestrial …